The use of spectrophotometric techniques for monitoring the level and variation of one or more parameters indicative of body condition is well known. Thus, U.S. Pat. Nos. 4,157,708 and 4,402,325 disclose ophthalmic devices incorporating one or more plethysmograph assemblies, but these devices have the disadvantage that light is assumed to be introduced along the axis of the pupil of the subject's eye.
In addition, U.S. Pat. Nos. 5,553,617 and 5,919,132 describe further non-invasive methods of measuring body conditions but, again, light introduced into the eye is assumed to be directed along the axis of the pupil.